This invention relates to slurry treatment, and particularly but not exclusively to the treatment of drill cuttings.
In the drilling of a well, drilling mud is pumped down the drill string to the drill bit, and the mud returns up the annular space between the drill string and the wall of the well, carrying with it the drill cuttings. In normal drilling, the solid cuttings comprise earth, rock and other materials of the strata through which the well is being drilled. In other instances, the drilling may be made into cement or even abandoned downhole tools and the drill cuttings may then include cement and/or metal pieces. The spent drilling mud carrying with it the drill cuttings is treated to separate out the drill cuttings before the drilling mud can be refused.
The separated drill cuttings, in suspension in drilling mud, are then transported away from the well to a plant for their thermal desorption or other disposal treatment. The transport of a drill cuttings slurry requires significant and expensive precautions to be taken to avoid spillage and, thus, environmental pollution. On land, drill cuttings slurries are transported by truck. For offshore drilling, the slurries are returned to land by skip, barge or other container and then transported by truck.
Offshore drilling is, for example, disclosed in xe2x80x9cNorwegian injection method uses separate use to wellheadxe2x80x9d, Offshore, April 1998, p84. This document discloses a method for dealing with drill cuttings from sub-sea formations. A drill cuttings slurry is formed on the off-shore rig, then pumped under pressure via a flexible riser back to the sub-sea wellhead for injection into the formation. This method avoids having to transport the cuttings to shore.
WO 93/20328 discloses a method and apparatus for processing drill cuttings, which method includes the formation of a solid-rich drill cuttings slurry which is then injected into a subsurface formation. The object of the cuttings processing system described in WO 93/20328 is to provide a satisfactory slurry for injection into the formation.
It will be appreciated that transfer of the slurries from a barge, for example, for a few feet (a few 0.3 m) to a truck can present risks of spillage but this risk has been much reduced by pumping the slurry from the barge into the truck on land. For this purpose, low pressure high flow pumps are used which can accommodate the solids content of drill cuttings slurries and move them the short distances involved. However, the subsequent transport by truck over longer distances is environmentally risky, as is the emptying or discharge of the slurry from the truck at its destination.
We have now found that surprisingly, drill cuttings slurries can be moved over extended distances by pumping, and this eliminates the need for the use of trucks and all the attendant spillage risks.
According to one aspect of the present invention, therefore, there is provided a method of transporting drill cuttings and the like for treatment or disposal away from the source of the slurry, which comprises pumping them with a high pressure pump through an extended pipeline.
According to a feature of the present invention, a high pressure pump is used to pump the drill cuttings through an extended pipeline. As is known, high pressure pumps can also be operated at lower pressures, and in accordance with the invention, the high pressure pump may be so operated to transport the drill cuttings. The important point is that it is a high pressure pump and is thus able to deliver high pressure pumping when needed or lower pressure pumping as appropriate. In this way, optimum pumping is available at all times during operation.
In general, for safety reasons, we prefer if possible to pump the drill cuttings through the extended pipeline at a low, rather than a high, pressure. In order to achieve low pressure transport, the slurry must be of an appropriate low viscosity: higher viscosity slurries require higher pressure pumping. The viscosity of a slurry can be reduced, if necessary, by adding fluids thereto, e.g. oil or drilling mud or other fluids.
A further possibility to assist low pressure flow is to pump a fluid into the extended pipeline at or downstream of the high pressure pump, and preferably immediately downstream thereof. The fluid is preferably introduced in the same general direction as the slurry flow in order to assist the flow of the slurry. The introduction of the fluid in effect lowers the viscosity of the slurry. We prefer to use air as the fluid, the air being injected from an air hose connected to a compressor. This will, of course, only be feasible if the air (or other fluid) supply is at a greater pressure than that prevailing at the injection point in the extended pipeline. In general, air pressures of up to about 100 psi (about 7 bar) can be used.
As will be well understood by those skilled in the art, the constitution of a drill cuttings slurry can vary widely. However, to be safely pumpable using a high pressure pump, it must be free of large solid lumps (e.g. greater than 5 mm in size). To this end, we prefer to pass the slurry through a suitable screen and/or one or more macerators to remove or crush any large solids, before the slurry passes to the high pressure pump.
Also, before being passed to the pump, the drill cuttings slurry is preferably homogenised to ensure optimum pumping. This can be achieved, for example, by stirring or otherwise mixing the slurry.
When high pressure pumping is used in the present invention, it would normally be at a pressure of at least 10 bar, whereas with the low pressure pumping would normally be below about 5 bar. High pressure pumps suitable for use in the present invention are commercially available, examples being Halliburton ST 400. We have found that, in accordance with the present invention, drill cuttings can be pumped considerable distances, for example from 50 m up to about 200 m or more. This is normally quite sufficient to transport the cuttings to a storage or treatment station, but if necessary one or more auxiliary pumps can be provided in the pipeline so as to extend the distance pumped.
We prefer that the drill cuttings slurry contain from 30 to 70% solids by volume, more preferably from 40 to 60%.
According to a further aspect of the invention, there is provided apparatus for pumping a drill cuttings slurry or the like, which apparatus comprises a tank for receiving the slurry to be pumped, preferably means for removing large solids from the slurry, preferably means for adding fluid to the slurry, means for agitating the slurry in the tank to mix it, and a high pressure pump for receiving slurry from the tank for pumping away from the tank.
Preferably, the means for removing large solids from the slurry is upstream of the tank so that raw slurry passes therethrough before reaching the tank. Suitable means include screens and/or macerators. Any large solids can be separated or crushed to smaller size, as appropriate. Most preferably, the screens or macerators are located on, e.g. on the suction side of, or otherwise incorporated with, a conventional low pressure pump which may, for example, be located on a barge carrying slurry from an offshore well. In this case, the screened and/or macerated slurry is then pumped under low pressure from the barge to the tank. Alternatively, the screens or macerators may be mounted above the tank so that treated slurry exiting the screens or macerators is delivered under gravity into the tank, although this is not preferred.
The fluid adding means preferably comprises one or more containers for oil, drilling mud or other fluid, for addition to the slurry as required to achieve a pumpable consistency. Advantageously, the container(s) can be mounted above the tank.
Agitation of the slurry contents of the tank is important to ensure a satisfactory feed to the high pressure pump. Without agitation, there can be settlement of solids in the tank which is undesirable. Agitation can be effected by, for example, mounting one or more stirrers in the tank, or in any other suitable manner.
The high pressure pump receives slurry from the tank and pumps it away, preferably at the lowest pressure at which the desired slurry transport can be efficiently effected. Preferably, the outlet from the tank to the pump is mounted at of near the base of the tank.
According to a feature of the invention, it is preferred that the apparatus be in modular form so that it can be transported to, and constructed on, site relatively easily. The modules may, for example, comprise the tank, the pump, the fluid storage container(s) and the macerators or screens.
Whilst the apparatus and method of the invention have been particularly described with reference to drill cuttings, they are also of utility with other similar slurries such as mine waste slurries.